As the cover stock of two-piece golf balls and some wound golf balls, ionomer resins in the form of ethylene/(meth)acrylic acid copolymers have been widely used and accepted because of their long-lasting impact resistance and cut resistance. Since the ionomer resins provide hard hitting feel and high hardness as compared with the balata rubber conventionally used as the cover stock, ionomer covered golf balls are difficult to impart a desired spin rate and inferior to control on iron shots.
For improvements in these respects, Sullivan, U.S. Pat. No. 4,884,814 or JP-A 308577/1989 proposes to blend a hard ionomer resin in the form of a zinc or sodium salt of an ethylene/(meth)acrylic acid copolymer having a certain spectrum of physical properties with a specific amount of a soft ionomer resin in the form of a zinc or sodium salt of an ethylene/(meth)acrylic acid/(meth)acrylate terpolymer. The soft/hard ionomer blend is used as a golf ball cover. This technique is quite effective for improving the hitting feel and control of golf balls using a conventional ionomer resin in the form of an ethylene/(meth)acrylic acid copolymer as the cover.
Nevertheless, the golf ball cover made of the above-mentioned blend of soft and hard ionomers of zinc or sodium salt type has several problems. Since the soft ionomer resin in the form of a zinc or sodium salt of an ethylene/(meth)acrylic acid/(meth)acrylate terpolymer is less resilient, the cover which is made softer using this soft ionomer resin is significantly reduced in restitution. In the manufacturing process, a molded part of this blend can be surface roughened by deburring and surface polishing because ionomers neutralized with different metal ions are less compatible with each. The cover becomes soft and improved in spin characteristics for the reason that the area of the ball in contact with the club upon iron shots is increased, which allows the cover surface to be scraped off by grooves across the iron club face, giving rise to the problem, known as a scuffing phenomenon, that the ball surface becomes fluffy. That is, the low compatibility between ionomers neutralized with different metal ions in a blend leads to a lowering of scuff resistance.
It is noted that U.S. Pat. No. 4,884,814 also discloses a blend of ionomers neutralized with the same metal ion, that is, a blend of sodium ion neutralized ionomers or a blend of zinc ion neutralized ionomers. According to our follow-up test, a blend of ionomers neutralized with the same metal ion as disclosed in this U.S. patent failed to provide a good balance of hitting feel (soft feel) and restitution.